With a view to sustainable effective utilization of marine organism resources, there have been adopted the TAC (Total Allowable Catch) system and the TAE (Total Allowable Effort) system, which are intended for major fishes to be fished.
In order to improve a management effect of resources by these systems, it is necessary to exactly assess the resource amount for each kind of fishes. As one of the means therefor, larval and juvenile fishes are collected according to water depth.
In order to collect larval and juvenile fishes in each layer, there have been conventionally proposed a trawl-net collecting apparatus referred to as “MOCNESS”, a trawl-net collecting apparatus referred to as “RMT”, and a trawl-net collecting apparatus referred to as “LOCHNESS”.
FIG. 3 generally shows a conventional LOCHNESS-type trawl-net collecting apparatus.
As shown in FIG. 3, the conventional LOCHNESS-type trawl-net collecting apparatus includes a rectangular cylindrical frame 22 for towing a plurality of trawl nets 21a, 21b, 21c, 21d. 
The frame 22 includes therein plural pairs of vertical bars 23.
A horizontal slide bar 24 is extended between each of the pairs of vertical bars 23, the slide bar 24 having opposed ends thereof that are slidably engaged with the vertical bars 23.
A net-mouth edge of a lower net of the first trawl net 21a is anchored to the slide bar 24 between the first pair of vertical bars 23, and a net-mouth edge of an upper net of the first trawl net 21a is anchored to another slide bar 24 between the second pair of vertical bars 23.
Anchored to the slide bar 24 between the second pair of vertical bars 23 are the net-mouth edge of the upper net of the first trawl net 21a, and a net-mouth edge of a lower net of the second trawl net 21b. 
Similarly, anchored to the slide bar 24 between the third pair of vertical bars 23 are a net-mouth edge of an upper net of the second trawl net 21b, and a net-mouth edge of a lower net of the third trawl net 21c. This is similar to the fourth and fifth slide bars 24. A net-mouth edge of an upper net of the trawl net and a net-mouth edge of a lower net of the trawl net adjacent thereto are anchored to the same slide bar.
The LOCHNESS-type trawl-net collecting apparatus 20 has, on an upper part thereof, locking devices 25 for locking the respective slide bars 24.
Outside the frame 22, floats 26 are disposed on opposed sides of the frame 22, so that a buoyancy of the LOCHNESS trawl-net collecting apparatus 20 can be adjusted.
Horizontal fin stabilizers 27 are disposed rearward the frame 22, so that a stance of the LOCHNESS-type trawl-net collecting apparatus 20 can be stabilized.
Net-towing wires 28 are connected to the upper parts of the LOCHNESS-type trawl-net collecting apparatus 20.
In all the conventional “MOCNESS”, the “RMT”, and the “LOCHNESS”, the net-towing wires are connected to the upper parts of the trawl-net collecting apparatus. Thus, there is a problem in that, when a net-towing speed is increased, a net mouth may be steeply inclined by a water flow.
In the MOCNESS, a net-towing speed has to be limited to about 1.2 meter/second or less. However, even when a net-towing speed is between 0.7 and 1.2 meter/second, the net mouth may be inclined at 40° or more.
In the RMT, not only a shape of the net mouth tends to be changed but also the net mouth tends to be steeply inclined depending on a stage that is opened and a net-towing speed. Thus, a net-towing speed has to be limited to 1.0 meter/second or less.
In the LOCHNESS, in order to restrain an inclination of the frame relative to a net-towing direction, a weight of the frame is increased, and horizontal fin stabilizers for controlling a posture of the LOCHNESS are provided. Thus, the structure is complicated, and the LOCHNESS is difficult to be handled because of its heavy weight.